In most ultrasound systems, raw echo data is subjected to many digital signal processing operations prior to being displayed or otherwise presented to a physician or ultrasound technician. The most common signal processing operation is filtering the data to remove spurious signals. For example, a wall filter is used in Doppler and color flow imaging modes to isolate the echoes caused by moving blood flow from those echo signals produced by tissue or moving vessel walls.
In any digital filter design, it is desirable to achieve good filtering characteristics, stability and ease of implementation. Most designs are therefore a compromise between these considerations. One problem affecting many designs is that the first few outputs of the filter are erroneous until the filter achieves its steady state operating condition. If supplied with a limited set of input data, these filter transients effect the number of valid data points that can be used to calculate the signals of interest to the physician. For example, the problem is particularly noticeable in color flow imaging where limited data is obtained to calculate blood velocity. In addition, the transient problem exacerbates a gap in Doppler signals that is caused by an ultrasound machine's alternating between different imaging modes.
While there are filter designs that do not produce transients, such as regression filters, these designs are generally computationally complex. In addition, it has been shown that regression filters produce some undesirable artifacts. See Hans Torp et al., "Clutter Rejection Filters in Color Flow Imaging: Theoretical Approach," IEEE Transactions on Ultrasound, March 1997, pp. 417-424.
Given the shortcomings in the art, there is a need for a digital filtering technique that is easy to implement and reduces the number of transients produced in order to maximize the number of valid output data for a limited set of input data.